Radiofrequency ashing

Within the past decade the technique of electronic (radiofrequency) low-temperature ashing has been used to investigate mineral matter in coal. In a low-temperature asher, oxygen is passed through a radiofrequency field, and a discharge takes place. Activated oxygen thus formed passes over the coal sample, and the organic matter is oxidized at relatively low temperatures—usually less than 150°C (14). 

Identification of Minerals in Coal. Once the low-temperature mineral matter residue has been obtained by radiofrequency ashing, the minerals can be identified, and their concentrations can be determined by a variety of instrumental techniques. The best developed, most inclusive, and probably most reliable method used thus far in distinguishing minerals in coal is x-ray diffraction analysis. It has been used extensively by Gluskoter (15), Wolfe (17), O Gorman and Walker (2), and Rao and Gluskoter (1) and has been somewhat successful in quantifying mineral analyses. 

Two types of coal ash samples have been prepared routinely for analysis at the Illinois Geological Survey. Low-temperature ash samples (12), in which the bulk of the mineral matter remains unchanged, are prepared by reaction of the coal with activated oxygen in a radiofrequency field. The effective temperature produced by this device is approximately 150 °C. Such samples were unsatisfactory for emission spectroscopic analysis. It is postulated that the presence of largely unaltered mineral matter, such as carbonates, sulfides, and hemihydrated sulfates (12), caused the observed nonreproducibility of results. High-temperature ash samples, prepared in a muffle furnace, consisted mainly... 

Factors that affect the rate of low-temperature ashing other than radiofrequency power and oxygen flow rate are the coal particle size and depth of sample bed. Typical conditions for ashing are a particle size of less than 80 mesh, a sample layer density of 30 mg/cm2, oxygen flow rate of 100 cm3/min, chamber pressure of about 2 torr, and a 50-W net radio-frequency power. The total time required is 36 to 72 hours, and specified conditions must be met during the procedure to obtain reproducible results. 

Low-temperature ashing (G1-G3) offers advantages over conventional dry-ashing techniques. A radiofrequency discharge is used to produce activated oxygen radicals, which are very reactive and will attack organic matter at low temperatures. This minimizes volatility and retention losses. Radiotracer lead in blood samples has been recovered quantitatively by this technique (G2). 

---